1. Field of the Invention
This invention relates to the scanning of imaged storage photo-stimuable phosphor plates, and more particularly to a process and apparatus for scanning imaged storage photo-stimuable phosphor plates and provided with automatic gain adjustment to consistently produce computer stored images of acceptable clarity.
2. Description of the Prior Art
Traditional x-ray imaging systems using silver-based films are being replaced by radiographic imaging systems using photo-stimuable phosphor imaging films or substrates eliminating the need for physical storage of the imaged film since digital retrieval of the image information includes input and computer storage permitting viewing of such information on a viewer, such as CRT tube assembly. Additionally, such computer stored image information permits facile electronic transmission to any location obviating physical transfer of the imaging film.
When ionizing radiation or x-rays impinge on such foils or photo-stimuable phosphor imaging plates, metastable storage centers are produced, which are lattice defects or color centers which have trapped a charge carrier (electron) produced by the ionizing radiation. Such storage centers are stable over long periods of time. If the storage centers are illuminated with a narrow laser beam of corresponding wave length, the storage centers will be moved into a higher excited state, from which the charge carriers can recombine with the emission of light referred to as photo-stimuable luminescence. The latter process is also referred to as the recombination of storage centers.
At such points of the storage foil or photo-stimuable phosphor imaging plate, whereon a larger amount of x-rays has impinged, one obtains by reading this point using a reading light beam, a higher amount of light quanta than at such points which have received only a few x-rays. If the storage foil or photo-stimuable phosphor imaging plate is scanned in two dimensions, the output signals of a light detector receiving the photo-stimulated luminescence corresponding to the optical density of a conventional x-ray film. In a reading device, such as a scanner, two dimensional scanning of the storage foils is obtained by arranging the storage foil or photo-stimuable phosphor imaging plate on the outer surface of a drum, and moving a reading unit along a generating line of the drum.
In U.S. Pat. No. 6,599,004 to Thoms, there is disclosed a apparatus for reading flexible storage foils wherein the flexible storage film having image-wise projected information is positioned on a cylindrically-shaped surface and is caused to be linearly moved while being interrogated or scanned via a slot by a light beam generated in a helical line in a continuous manner from a point disposed at the axis of the cylindrically-shaped surface. At the conclusion of the reading process, the flexible storage film is caused to be returned to its starting position with the thus read storage film being thereafter separately processed to remove any latent image by exposure to light to cause trapped charged electrons to be dissipated thereby restoring the storage film for use in a subsequent receiving projected imaging information process. In the event that the thus produced image is vague, faded or not as detailed as desired, the imaged plate may be reprocessed in the apparatus with gain adjustments to achieve the desired clarity.
In U.S. Pat. No. 6,762,430 to Alzner, et al. and assigned to the same assignee as the present invention, hereby incorporated by reference, there is described a plate guide system for dental imaging plates for use in the hereinabove described apparatus of U.S. Pat. No. '014 to Thoms. Following scanning, the imaged plate is further passed through an erasing assembly whereby the imaging capability is thereby restored. In the event that the resulting image is vague, faded or is not as detailed as desired, the once exposed area of the patient must be x-rayed and the thus produced imaged storage plate is again scanned at an appropriate gain level.